The proposal is designed to obtain a better understanding of the reaction of uterine tissues to the IUD in order to help elucidate the mechanism(s) of action of the device and to understand the prolonged effects of the IUD. To accomplish these objectives the following studies are planned: 1. To determine the role of exogenous neutrophils on the antiimplantation effect of the IUD, 2. To determine the relationship between prostaglandin and the antiimplantation effect of the IUD, 3. To determine the role of histamine and mast cells on the antiimplantation effect of the IUD, 4. To study the estrogen-like effect of the IUD following short-term exposure of the uterus to the foreign device, 5. To determine whether the ovarian hormones influence the metaplastic and neoplastic alterations observed in uteri bearing an IUD for prolonged periods of time, 6. To study the reversibility of the uterus to a functional organ following removal of the IUD. In the experiments the effect of the polyethylene and silk IUDs will be compared by studying specific cells and tissues of the uterus using histological, histochemical, autoradiographic and electron microscopic procedures. Since the polyethylene device does not make the entire horn refractory to the blastocyst, it serves as an excellent model to compare the tissue in the same horn - the IUD area and the non-IUD area.